Bad news for oxygen breathers: Our planet has been losing atmospheric oxygen for at least the last million years.

It sounds worse than it is: Earth's atmosphere is steadily losing oxygen. But before you panic and gasp for breath, understand that oxygen levels have only dropped by 0.7 percent over the past 800,000 years. So you don't have to worry about widespread asphyxiation just yet. Still, it's an alarming find that scientists aren't quite sure how to explain.

In the study, researchers were able to measure atmospheric oxygen levels over time by analyzing tiny air bubbles trapped in ice core samples taken from Greenland and Antarctica. The research was published in the journal Science.

“We did this analysis more out of interest than any expectation,” said Princeton University geologist Daniel Stolper, to Gizmodo. “We didn’t know whether oxygen would be going up, down, or flat. It turns out there is a very clear trend.”

Though oxygen is trending down, there's still plenty to breathe; ecosystems shouldn't be affected anytime soon. Even so, scientists will want to investigate the cause in order to know exactly what we should expect moving into the future. Also, it's worth probing what effects human impacts might have on long-term oxygen levels.

Interestingly, this isn't the first time that Earth's oxygen levels have fluctuated. For the first few billion years of history, our planet actually didn't have any oxygen at all. It wasn't until the evolution of tiny green algae called cyanobacteria, which produced oxygen through photosynthesis, that our air was pumped full of the stuff. The further evolution of plants meant even more oxygen, until levels peaked at around 35 percent (they are at around 21 percent today) during a period called the Carboniferous. In fact, oxygen levels were so high during this period that it allowed many arthropods — insects particularly — to grow to mammoth sizes, some with wingspans over two feet in length.

Lower oxygen levels today might mean smaller insects — that's probably a relief for many people — but we don't want oxygen to get too low. So what gives? Researchers offered a few theories.

The first theory has to do with erosion, which scientists believe has sped up in recent geologic history. More erosion means more fresh rocks are being exposed to the air, and rocks can suck up a lot of oxygen via oxification. Another theory has to do with climate change, but not the human-induced kind. Until our recent warming trend, Earth's average temperature had been dropping for several million years. Colder temperatures increase the solubility of oxygen in the oceans.

But even though the planet's temperature has been increasing over the last century, this warming trend is not likely to help on the oxygen front. That's because we're consuming oxygen at a rate that's a factor of a thousand times faster than before.

So perhaps oxygen levels are still dropping and will continue to drop so long as human activity persists, and so long as human activity has profound environmental effects. Scientists will need to do more research to know for sure, though.

“It’s yet another indication of our collective ability to do what happens [naturally] on the Earth, yet so much faster,” expounded Stolper.